1. Field of the Invention
This invention relates to a method for protecting a metallic surface from corrosion, more particularly, to a method for forming on the inner surface of a copper alloy condenser tube a film of an iron oxide, which film serves to protect the inner surface from corrosion.
2. Description of the Prior Art
A film of an iron oxide such as ferric oxyhydroxide (FeOOH) pertains excellent properties as a corrosion protective film for the inner surface of a copper alloy condenser tube adapted for use in a heat exchanger. In a representative method for forming such a film, ferrous ions are incorporated in sea water as a coolant and the thus-prepared ferrous ion-containing sea water is fed into a condenser tube to form a chemically produced protective film on the inner surface thereof. However, this method is accompanied by a drawback that the film-forming speed is extremely slow, coupled by a problem from the viewpoint of environmental protection that water containing iron ions are discharged. Another method is disclosed in Laid-open Japanese Patent Specification No. 71697/1977 laid open to the public on June 15, 1977; naming as the applicant Nippon Electric Co., Ltd., and as the inventors Shigeo Noguchi, Kozo Nishimoto, and Morimasa Nagao; and entitled "PREPARATION METHOD OF THIN MAGNETIC FILM OF IRON OXIDE", in which, after immersing a substrate in an aqueous acidic solution, the solution is heated to 50.degree.-80.degree. C. while blowing an oxidizing gas thereinto, and iron powder is then thrown into the solution. However, this method is not practical as no protective film would be formed at all on a copper alloy substrate or the film-forming speed is extremely slow.
In view of the above-described drawbacks of the conventional methods, the present inventors have proceeded with various researches with a view toward developing a method capable of forming efficiently a corrosion-protective film of an iron oxide on the inner surface of a copper alloy condenser tube in a simple operation. As a result, it has been found unexpectedly that the above object would be achieved fully by adopting a new method which will be described below, resulting in the completion of this invention.
The principal feature of a method for protecting an inner surface from corrosion in accordance with this invention resides in applying a thin layer of an acidic suspension containing iron powder onto the inner surface of a copper alloy condenser tube and exposing said thin layer to an atmosphere of an oxidizing gas to form a film of an iron oxide on said inner surface.
The present invention is similar to the method disclosed in the above-referred Laid-open Japanese Patent Specification No. 71697/1977 in that both inventions use an acidic solution or suspension containing iron powder and an oxidizing gas. However, in the present invention, the inner surface of a condenser tube is exposed to an atmosphere of an oxidizing gas while being applied with a thin layer of the acidic suspension containing iron powder and the formation of a protective film can be performed at an extremely high speed. Such a fast formation of the protective film may be attributed to the following reasons. Namely, iron particles contained in the suspension have many chances to be brought into contact with the copper alloy base. As shown in FIG. 1, an iron particle 1 and the copper alloy base 2 act as an anode and cathode respectively, and a colloidal particle 3 of resultant iron oxyhydroxide or the like (charged positive under mild acidic conditions) is electrostatically attracted onto the surface of the copper alloy base 2, thereby achieving an extremely fast formation of a film of iron oxyhydroxide or the like and also enhancing the adhesion of the thus-formed film to the base. The numerals 3 and 4 are respectively a thin layer of an acidic suspension and an oxidizing gas atmosphere.